The present patent concerns a nozzle design which reduces jet exhaust noise. Among the sources of noise of turbojet and turbofan engines, that corresponding to the jet exhaust is one of the most important. The jet exhausts into the atmosphere, where ambient air moves at much lower speed, particularly in low-speed flight phases, like take-off and landing; these flight phases are of most concern regarding noise around airports, and are subject to ICAO (International Civil Aviation Organisation) noise certification rules, as well as additional restrictions at some airports.
The jet exhausting into ambient air forms a shear layer, across which the velocity changes from the jet speed to the ambient speed. The noise from sources inside the jet is transmitted to the exterior across this shear layer, and this process can change the intensity, directivity and spectrum of sound, thereby reducing the noise disturbance outside. In the case of a turbofan, there are two co-axial jets, and thus two shear layers; the noise from sources in the inner jet is transmitted to the exterior across two shear layers, with possible multiple internal reflections, leading to greater scattering effects and lower noise. The various mechanisms of noise reduction are explained next, by starting with simple models of the shear layer, and elaborating them step-by-step, until close similarity to a real shear layer is achieved.
The simplest model is a vortex sheet, i.e. a surface across which the velocity changes abruptly from the jet speed to the ambient speed. This model would apply if the wavelength of sound was much larger than the thickness of the shear layer, and the scale of irregularities of the shear layer and if no turbulence were entrained with the shear layer. All these restrictions will be removed later. Staying for the moment with a plane vortex sheet, it is clear that a sound source inside the jet will emit an acoustic wave, incident upon the vortex sheet, giving rise to reflected and transmitted waves. The existence of a reflected wave, means that not all acoustic energy is transmitted across the shear layer, so there is a reduction in acoustic intensity outside the jet. The transmitted wave is not radiated in all directions, i.e. there is a xe2x80x98zone of silencexe2x80x99 outside the jet, where there are only evanescent waves. Thus the transmitted sound field has a reduced intensity compared to the direct sound field of the source, and also a modified directivity pattern including one xe2x80x9czone of silencexe2x80x9d or two (the latter for jet Mach numbers exceeding two).
Let""s elaborate the model a little more, and assume that the vortex sheet is not flat but rather is irregular. This will apply if the wavelength is still much larger than the thickness of the shear layer but is not much larger than the scale of its irregularities; for the moment, turbulence or eddies entrained with the shear layer are not considered yet. The incident sound wave originating from the source now hits the irregular vortex sheet at different heights, and gives rise locally to reflected and transmitted waves. Since these waves are no longer in phase, there can be destructive interference in the transmitted sound field, reducing the noise level outside the jet, compared with a flat vortex sheet between the same media. Also, the irregular vortex sheet can transmit sound into what would be the xe2x80x98zone of silencexe2x80x99 of a flat vortex sheet, because local scattering conditions may allow this; thus the transmitted acoustic energy is spread over a wider range of directions, by an irregular vortex sheet, as compared with a flat vortex sheet, further modifying the directivity, and reducing the intensity of radiation.
It should be borne in mind that the irregular vortex sheet separating the jet from ambient moves at a convection velocity intermediate between the two. When sound is scattered from a moving surface, its frequency is changed by the Doppler effect. Besides the Doppler shifted frequency of the source, harmonics at other frequencies may appear. Thus a moving, irregular vortex sheet further reduces the noise relative to a static irregular vortex sheet, also because the acoustic energy is spread over more frequencies.
So far it has been assumed that the moving irregular vortex sheet representing the shear layer has a fixed shape; actually, real jet shear layers are turbulent, and thus should be represented by a randomly irregular moving vortex sheet if the wavelength is very large compared with the thickness of the shear layer. This vortex sheet with randomly changing shape causes random changes in the direction of the transmitted wave, and thus scatters sound over a wider range of directions and causes more interference between wave components, also a randomly irregular moving vortex sheets causes random Doppler shifts, thus spreading the acoustic energy over a wider spectrum. The three effects, viz:(i) more interference between wave components, (ii) wider directivity pattern and (iii) broader spectrum, all contribute to reduce the acoustic intensity received on average on each direction and the each frequency outside the jet.
It is appropriate to introduce at this stage the spectral directivity I(xcex8,xcfx86,xcfx89) defined as the acoustic power dW received per unit frequency dxcfx89 and unit solid angle dxcexa9
dW=I(xcex8,xcfx86,xcfx89)dxcexa9,
where the solid angle dxcexa9 is expressed in terms of spherical coordinates (xcex8,xcfx86) by:
dxcexa9=sin xcex8dxcex8dxcfx86.
It has already been explained why a randomly irregular moving vortex sheet reduces the spectral directivity and total acoustic power received from the source. The explanation has been based on three scattering effects: (i) wave interference; (ii) wider directivity pattern; (iii) spectral broadening. These effects have been demonstrated for scattering by a randomly irregular moving interface, which is an adequate model of a shear layer if the wavelength is much larger than the thickness of the shear layer. We shall now lift this last remaining restriction.
For sound of arbitrary, i.e. smaller wavelength, the shear layer no longer appears as a discontinuity of velocity between the jet and ambient medium, but rather as a smooth velocity change or shear flow. The transmission of sound through this shear flow demonstrates scattering effects, like for a vortex sheet, with the additional possibility of sound absorption by the flow, at critical points where the Doppler shifted frequency vanishes. This is a further noise reducing mechanism.
A real shear layer is not only a shear flow, but also entrains turbulence and eddies. The effects of turbulence and eddies on the scattering of sound are similar to those of a randomly irregular moving interface, in the sense that: (i) there is backscattering, i.e. some of the acoustic energy is scattered back into the jet, and thus does not reach the ambient medium; (ii) there is a wider directivity pattern, because sound is deflected into directions which might not be present in the incident wave; (iii) there is spectral broadening, because the convection of sound by turbulence and eddies causes random Doppler shifts; (iv) the changes in direction of propagation and frequency cause interference between sound waves. The ensemble of four effects (i) to (iv), plus possible (v) sound absorption at critical layers is called xe2x80x9cscattering effectsxe2x80x9d in what follows. It is clear that these scattering effects reduce the noise transmission from a jet to the exterior.